Update peerDeps to allow prettier v4.

[cpojer]

I've been using the v4 alphas of prettier for a very long time, and they have always worked just fine with this plugin.

[IanVS]

Thanks Christoph, I also use v4 alpha of prettier, I should have done this long ago.

Are you sure that 4 will pick up the alpha versions? I don't 100% trust the semver calculator at https://semver.npmjs.com/, but it indicates that we'd need to use something like ^4.0.0-0. I know peer deps are sometimes treated differently from standard deps, but maybe that would be safer?

[cpojer]

Honestly, I don't know 🥲

[cpojer]

I think it worked!